Microstrip filters



Dec. 1, 1959 r 1'. E. HATTERSLEY 2,915,716

MICROSTRIP FILTERS Filed Oct. 10, 1956 GROUND PLATE 2 Q D Z LL] ,2 4 jI4 FREQUENCY INVENTOR.

THOMAS E. HATTERSLEY ATTORNEY United States Patent MICROST'RIP FILTERSApplication October '10, 1956, Serial No. 615,083

' 1 Claim. Cl. 333-73 This invention relates to microwave circuits, andis particularly directed to filters constructed by printed circuittechniques. 3

The design of filters with lumped reactance elements has long since beenstandardized, but the frequencies contemplated are in the longerwavelengths compared to the so-called microwave bands Where thewavelengths appreach the physical dimensions of the reactance elementsto be used. The introduction of transmission lines such as wave guides,coaxial cables, parallel wires, and the like, have facilitated thetransporting of microwave energy from one point to another with limitedcontrolled radiation, but has aggravated the problems of filter design.Obviously, lumped reactances connected to or near such lines causediscontinuities which disturb wave motion and generally destroy the verycharacteristics for which the lines were selected.

The object of this invention is to provide an improved filter forelectric or magnetic wave energy in the microwave bands of frequencies,and in which wave motion phenomena of the resonant transmission lines isfully utilized.

"Ice

adapted to the high frequency use contemplated here. Teflon ispolyte'traflubrethylene. "Formica"'a1id Resolite, sometimes Resilyte,are usually either melamine formaldehyde or phenolformaldehyde employedas a The objects of this invention are attained in a transmission linecomprising a sheet of insulating material of uniform predeterminedthickness and dielectric constant, having a ground plate of extendedarea adhered to one face of the sheet, and having a thin strip conductorof small cross section and extended length adhered to the other face toproduce a resonant transmission line. The filter is characterized bytabs of thin metal adhered to said other face and integrally joinedalong their edges to said line conductor, said tabs being of extendedarea to present a relatively large lumped capacity with said groundplate, compared with the capacity of the connected portion of said line.Some of the tabs are in turn integrally joined to thin metal stubsadhered to said other face of the insulating sheet, and so terminated attheir outer ends as to present lumped inductive reactance in parallelwith the capacitive reactance of the tabs.

Other objects and features of this invention will become apparent tothose skilled in the art by referring to specific embodiments describedin the following specification and shown in the accompanying drawing, inwhich:

Fig. l is a partly sectioned plan view of a filter embodying thisinvention,

Fig. 2 is a circuit diagram which is the equivalent of the circuit ofFig. 1,

Fig. 3 is a cross sectional view of the device taken on line 33 of Fig.1, and

Fig. 4 is a graph showing typical frequency characteristics of thefilters of this invention.

In Fig. 1 is shown the sheet 1 of insulation, having a relatively highdielectric constant and low high-frequency loss characteristics. Thesheet is of extended surface area, and is quite thin. Polystyrene,polyethylene, or insulators commercially known under the trademarks ortrade names Formica or Teflon or Resolite are well filler for cotton,asbestos, glass, 'or cellulose fabrics. I To one side of the "sheet isadhered a thin metal ground plate 2. The plate 2 is large in surface'area and underlies and extends well beyond the boundary of the circuitsprinted on the other face of the sheet. The ground plate, for economicand electrical reasons, may conveniently comprise copper foil firmlybonded as by a thermosetting adhesive to the one side of the insulatingsheet.

To the other side of the sheet is the strip conductor 3, which also maybe coppei foil, tailored to the desired size and shape and glued firmlyto the face of the sheet. Alternatively, the metal of strip conductor 3'may be prepared by applying an ink of powdered metal appropriatelypainted on the sheet and baked in a reducing atmosphere to bond themetal to the sheet in low resistance strips. A third and morecommonalternative comprises cladding overall the front side of the sheet,protecting selected portions of the clad jacket with a photoresistexposing to strong light, and etching away the undesired portions of thejacket. 7

According to an important feature of this invention, tabs 4 and 4a areformed on the insulating sheet integrally with the conductor 3, and arespaced along the conductor as shown. The tabs are shown in pairs,symmetrically on either side of the conductor 3, although symmetry isnot indispensable. The spacing between the tabs, and the length of theconductor 3 between the tabs, are preferably such that at thecontemplated operating frequency the sections of the line conductorappear as inductive reactances between the tabs. Accordingly, linesections 3a comprise finite inductances in series with line 3; withcapacities 4 connected in shunt to the line, as shown in Fig. 2. Aplurality of series inductances and parallel capacities comprise theladder of a low pass filter, attenuating all frequencies above thefrequency determined by the length of sections 3a and the areas of tabs4. The cutoff frequency, f Fig. 4, of such a filter is proportional toR1rL or 1/1rCR, where L is the series inductance of each section, C isthe adjacent shunt capacity and R is the load resistance at thereceiving end of the filter.

The low pass filter sections, 4 and 3a, are connected in series with aband pass filter comprising the capacities of tabs 4a. For band passcharacteristics, effective inductive reactances must be coupled inparallel to the capacitive reactances of the tabs 4a. For this purposeaccording to this invention, stubs 5 are formed of metal foil on theface of the sheet, integrally joined at one end to the edge of the tabs4a and appropriately terminated at their other or outer ends. Accordingto this invention the stubs are short circuited at their outer ends tothe ground plate 2. Eyelets 6 in the example shown are driven throughthe sheet to electrically connect the ground palte 2 to the outer endsof the stubs, as best shown in Fig. 3. The stubs are of such a length,respectively, that they present a predetermined effective inductivereactance to the edge of the connected tabs. This length is preferablyless than about wavelength of the shortest wave of the band to bepassed.

Each section of either filter may be selected in its constants bycomputing the surface areas of the tabs 4 and 4a for a given insulatorand adjusting the lengths of stubs 5 to alter the width of the pass bandand to shift the cutoff frequency of the low pass filter. If 1; is thelower frequency limit of the pass band and f is the upper limit of thepass band, see Fig. 3, and L is the series inductance and L is the shuntinductance, and the several capacities, C, are equal, with a loadresistance R, then the relationships of the parameters of the band passfilter of this invention may be simply stated as: L =R1r(f +f and L (f f)R/41rf Accordingly, the width of the pass band is convenientlyadjusted, and the spacing thereof from the cutoff frequency of the lowpass filter is easily controlled. A band pass has been constructed tocover a two-to-one frequency range, and with the low pass filter incombination therewith, harmonic components and spurious responses areeffectively suppressed. The pass band f to f and the low pass, fcharacteristics are separately shown in Fig. 4 before combining.

To determine the inductance and capacities as a practical matter it isconvenient to assume a lossless transmission line and to employ basictransmission line formulas such as L ZVE where v is velocity ofpropagation, Z is characteristic impedance, L is inductance and C iscapacitance. Inductance in henries per meter may be obtained directlyfrom Z /v, while capacity in farads per meter may be obtained from l/Z vto accommodate the series and shunt elements of a filter of any givencharacteristics. Of course, the wider the line 3, the greater is thecapacities per unit length of the line.

The formed tabs and stubs do not present discontinuities to the evenlydistributed constants of the transmission line 3, and present very lowinsertion losses to the line. The physical thickness of the filters,comprising the sheet 1 and metal foil parts 2 and 3, may be but a fewthousandths of an inch, thus requiring volumetric space extremely smallcompared to the space required of filters composed of the usual lumpedelements. Filter cards of the type shown are readily replaceable in achassis, should substitution of frequency characteristics be desired.Extensive experimentation has shown that the electrical characteristicsof the filters of this invention are extremely stable with age and withenvironmental changes.

While a specific embodiment of this invention has been and shown anddescribed, other modifications will readily occur to those skilled inthe art. It is not, therefore, desired that this invention be limited tothe specific a-rrangement shown and described, and it is intended in theappended claim to cover all modifications within the spirit and scope ofthis invention.

What is claimed is:

A band pass filter comprising a sheet of insulating material, a groundplate of extended area adhered to one face of said sheet, an elongatedstrip conductor of limited area adhered to the other face of said sheet,spaced foil-like tabs adhered to said other face and electrically joinedto said strip conductor, said tabs being of extended area providinglumped capacities with said ground plate larger than those of theconnected portions of said strip, and foil-like stubs of less width thansaid tabs adhered to said other face and electrically connected to saidground plate and at least some of said tabs, the length of said stripsbetween the tabs being adjusted so that the inductive reactance Lthereof is proportional to R/1r(f +f and the length of said stubs, for aselected termination, being adjusted so that the inductive reactance, Lthereof is proportional to (f f )R/1rf where R is the resistance of theload at the end of said strip, and where f and f are, respectively, thelower and upper frequency of the desired pass band. sired pass band.

References Cited in the file of this patent UNITED STATES PATENTS2,411,555 Rogers Nov. 26, 1946 2,558,748 Haeff July 3, 1951 2,751,558Grieg et al. June 19, 1956 2,760,169 Engelmann Aug. 21, 1956 2,819,452Arditi Jan. 7, 1958 2,820,206 Arditi Jan. 14, 1958 OTHER REFERENCESRadio-Electronic Engineering, September 1951, pages 16 and 31.

Electronics, vol. 27, No. 9, September 1954, pages 148-450.

